Ethylene and propylene polymers are produced commercially in the gas or liquid phase by contacting the monomer or monomers with a suitable catalyst. The conditions under which the polymerization is conducted depend upon the particular process employed. For example, high density polyethylene is generally produced at low temperatures and pressures, usually in the range of about 75.degree. to 90.degree. C. and 5 to 10 bar, respectively, while low density polyethylene is produced at high temperatures and pressures, such in the range of about 150.degree. to 250.degree. C. and 150 to 200 bar. Linear low density polyethylene is produced at moderate temperatures and pressures. The feed generally comprises high purity ethylene, and may include other alkenes, depending upon the product properties sought. Similarly, propylene polymerization is carried out at temperatures of about 15.degree. to 100.degree. C. and pressures of about 25 to 50 bar. In both of the above-mentioned processes it is desirable that the purity of the monomeric feed used in the polymerization process be very high, e.g in the neighborhood of 99.5% or more weight percent pure ethylene or propylene, as the case may be. The polymerization process is desirably continuous with the conversion per pass being less than 100%, and often as low as about 7 to 36%.
The alkene monomer, obtained from crude oil during refining, contains considerable amounts of the corresponding alkane, i.e. ethane or propane. The alkene is generally separated from the propane commercially by distillation. Since the boiling points of ethylene and ethane, and propylene and propane are close together, it is difficult and costly to produce polymerization grade ethylene and propylene by distillation. Furthermore, since the adsorption characteristics of the alkenes and corresponding alkanes are similar, it has previously been very difficult to produce suitable ethylene and propylene polymerization feed stock by adsorption.
The difficulty of separation of ethylene from ethane and propylene from propane causes a further complication with respect to polymerization processes. Since the polymerization is generally conducted at less than 100% monomer conversion, it is in the interest of economy to conduct the process on a recycle mode, with unreacted monomer being recycled to the polymerization reactor. Ethane and propane are not affected by the polymerization catalyst; therefore the concentration of ethane or propane in the system would gradually build up as the polymerization proceeds, if measures were not taken to prevent such buildup from occurring. Since efficient and cost effective ethylene-ethane and propylene-propane separation techniques were not previously available, one method of preventing ethane and/or propane from building up in polymerization systems was to continuously or periodically purge a portion of the gaseous polymerization reactor effluent from the system. Unfortunately, part of the valuable alkene monomer was also discharged from the system during the purge.
Continuous efforts are underway to enhance the efficiency of recycle ethylene and propylene polymerization processes. These efforts include investigations for improved procedures for purifying ethylene and propylene feed stock and for separating ethylene and propylene from ethane and propane, respectively, prior to recycling unreacted monomer to the polymerization reactor. The present invention provides such an improved procedure.